mizan-webview-transport + webview-channels: VSCode webview as Mizan frontend

Two new frontend packages let a VSCode webview consume a Mizan backend
through its postMessage channel — peer transports to `mizan-tauri-transport`
and the default `httpTransport()`.

- `@mizan/webview-transport` implements `MizanTransport` (call/fetch)
  over postMessage with correlation-id pairing. Drop-in for `configure({
  transport: webviewTransport() })`; codegen output and React adapter
  are unchanged.

- `@mizan/webview-channels` mirrors mizan-react's WebSocket-based
  ChannelConnection — RPC + subscribe over the same postMessage channel
  for long-running ops where short request/reply isn't enough.

Both expect an extension-host-side dispatcher that reads envelopes via
`webview.onDidReceiveMessage` and routes them through mizan-ts's
`handleMutationCall` / `handleContextFetch`. First consumer is the
holomorphic VSCode extension.

mizan-codegen: new `[source.script]` generic source. Spawns an arbitrary
command and reads stdout as KDL IR. Keeps mizan-codegen out of the
business of knowing every possible backend language while preserving
the "subprocess emits KDL" contract every other source already follows.
Holomorphic uses it to invoke `python -m holomorphic.emit_ir` against
the mizan_core registry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontends/mizan-webview-transport/src/index.ts

@@ -0,0 +1,141 @@
+/**
+ * @mizan/webview-transport — routes Mizan calls through a VSCode webview's
+ * postMessage channel instead of HTTP fetch or Tauri IPC. Paired with the
+ * extension-host-side Mizan dispatcher (e.g. `MizanHost` in the holomorphic
+ * extension), which receives envelopes via `webview.onDidReceiveMessage`
+ * and posts responses back via `webview.postMessage`.
+ *
+ * Usage (webview side, inside the bundled React/TS app):
+ *
+ *   import { configure } from '@mizan/base'
+ *   import { webviewTransport } from '@mizan/webview-transport'
+ *
+ *   configure({ transport: webviewTransport() })
+ *
+ * The transport keeps the same protocol surface as the HTTP and Tauri
+ * transports (call/fetch envelopes, {result, invalidate, merge} response
+ * shape), so the codegen output and React adapter are unchanged — only
+ * the wire channel differs.
+ *
+ * Envelope shapes (this side ↔ extension-host):
+ *
+ *   webview → host:
+ *     { kind: 'call',  id, fn, args }
+ *     { kind: 'fetch', id, context, params? }
+ *
+ *   host → webview:
+ *     { kind: 'response', id, ok: true,  body }
+ *     { kind: 'response', id, ok: false, error: { status, body } }
+ *
+ * Correlation by `id` lets multiple in-flight calls share the one
+ * postMessage channel.
+ */
+
+import { MizanError, type MizanCallResponse, type MizanTransport } from '@mizan/base'
+
+// VSCode's webview API — present at runtime, declared globally so the
+// transport can be authored without pulling vscode types into a generic
+// frontend package. Returned by acquireVsCodeApi() exactly once per
+// webview load; subsequent calls throw.
+declare global {
+    function acquireVsCodeApi(): VsCodeApi
+}
+
+interface VsCodeApi {
+    postMessage(msg: unknown): void
+    setState(state: unknown): void
+    getState(): unknown
+}
+
+type Pending = {
+    resolve: (data: unknown) => void
+    reject: (err: unknown) => void
+}
+
+type CallEnvelope = { kind: 'call'; id: string; fn: string; args: Record<string, any> }
+type FetchEnvelope = { kind: 'fetch'; id: string; context: string; params?: Record<string, any> }
+type Envelope = CallEnvelope | FetchEnvelope
+
+type ResponseOk = { kind: 'response'; id: string; ok: true; body: any }
+type ResponseErr = { kind: 'response'; id: string; ok: false; error: { status: number; body: string } }
+type ResponseEnvelope = ResponseOk | ResponseErr
+
+let _api: VsCodeApi | null = null
+
+function api(): VsCodeApi {
+    if (_api === null) {
+        if (typeof acquireVsCodeApi !== 'function') {
+            throw new Error(
+                '@mizan/webview-transport: acquireVsCodeApi is not defined. ' +
+                'This transport runs inside a VSCode webview only.',
+            )
+        }
+        _api = acquireVsCodeApi()
+    }
+    return _api
+}
+
+const pending = new Map<string, Pending>()
+let installed = false
+let counter = 0
+
+function install(): void {
+    if (installed) return
+    installed = true
+    window.addEventListener('message', (e: MessageEvent) => {
+        const msg = e.data as ResponseEnvelope | undefined
+        if (!msg || msg.kind !== 'response') return
+        const pen = pending.get(msg.id)
+        if (!pen) return
+        pending.delete(msg.id)
+        if (msg.ok) {
+            pen.resolve(msg.body)
+        } else {
+            pen.reject(new MizanError(msg.error.status, msg.error.body))
+        }
+    })
+}
+
+function nextId(): string {
+    return `mz_${++counter}_${Date.now()}`
+}
+
+function send<T>(env: Envelope): Promise<T> {
+    install()
+    return new Promise<T>((resolve, reject) => {
+        pending.set(env.id, {
+            resolve: resolve as (data: unknown) => void,
+            reject,
+        })
+        api().postMessage(env)
+    })
+}
+
+/**
+ * Build a Mizan transport that routes through a VSCode webview's
+ * postMessage channel. Install via:
+ *
+ *   import { configure } from '@mizan/base'
+ *   import { webviewTransport } from '@mizan/webview-transport'
+ *   configure({ transport: webviewTransport() })
+ */
+export function webviewTransport(): MizanTransport {
+    return {
+        async call(fn, args): Promise<MizanCallResponse> {
+            return send<MizanCallResponse>({
+                kind: 'call',
+                id: nextId(),
+                fn,
+                args,
+            })
+        },
+        async fetch(context, params) {
+            return send<any>({
+                kind: 'fetch',
+                id: nextId(),
+                context,
+                params,
+            })
+        },
+    }
+}